The present invention relates to a method of plotting in color with photoexposure heads and a color film having maximum exposure capacities in each of the color sensitive emulsion layers. More particularly, the present invention relates to a method of of controlling the color exposure of the film in a plotting operation to produce sharp, clear and accurate plots in multiple colors.
In a co-pending application Ser. No. 038,452 filed May 14, 1979 and entitled "COLOR PLOTTING DEVICE" by Webster and Larsen, a photoplotter is disclosed for producing colored plots on a photographic film having color sensitivity. An optical exposure head in the plotter projects beams of light having different or changeable color components onto the photosensitive surface of the film, and various emulsion layers in the photosensitive surface record the projected colors. Drive motors within the plotter move the spot of light generated by the colored beams over the film surface and expose the film in accordance with the color components found in the beam and spot.
The color plotting process may be advantageously employed in the fields of cartology, microcircuit manufacturing and other fields of use. Color plotting adds new dimensions to the field of plotting that were not previously available, and correspondingly increases the end uses of a plotter by many magnitudes.
The exposure of a color film in a plotting process differs in at least one major respect from the exposure of the same film through a camera shutter. In a camera all of the film is exposed in a single instant when the shutter is momentarily open. The degree of exposure of the film is controlled by preset aperture and time settings so that no portion of the film emulsions receives more exposure than can be tolerated for accurate and clear presentation of the color image when developed. The settings to control exposure are based upon the general level of light reflected from the object being photographed when illuminated by artificial or natural light.
In a photoplotter, however, plotting is carried out in low light level conditions, and the spot of light produced by a color beam projected onto the film exposes a very limited area at any instant in time. Only by moving the spot over the film does one progressively expose a complete plot pattern. The ability to move the light spot over the film surface adds flexibility to the plotting process and allows any portion of the photosensitive surface to be exposed to the exclusion of the rest of the film. Additionally, any one position on the film surface can be exposed several times by repetitively locating the spot over that position with the same or different colors providing exposure. As a result of the flexibility in both the movement of the spot and the color composition of the spot, control of the exposure presents new problems not encountered in camera photography or photoplotting with black and white film.
First of all, one advantage of a color plot is that a different element in the plot can be represented by different colors and the elements can overlie one another without loss of discrimination. Generating overlying colors means that one position of the film is exposed two or more times by passing the projected light beam over that position repeatedly with the same or different color components in the beam. When the spot of light generated by the projected beam exposes the film on two or more occasions, the possibility of over-exposure is presented even though the intensity of each light spot at a position is within the original tolerance of the film emulsions.
Additionally, color films regardless of their types include on the photosensitive surface three different emulsion layers which record the color components of a colored light spot in terms of the primary color constituents, that is, those primary colors from which all other colors may be defined. Typically a color film has one emulsion layer which records all colors having any red constituents, another emulsion layer for recording all blue constituents, and a third emulsion layer for recording all green color constituents. The primary colors red, blue and green may be added together in equal proportions to form white light, and for this reason are known as the additive primaries. In practice the exposed and developed film defines the recorded colors in terms of the subtractive primary colors cyan, magenta and yellow which are absorbed as dyes in the respective emulsion layers during the developing process. The subtractive primary colors cyan, magenta and yellow are the complements respectively of the primary colors red, green and blue, and are called subtracprimaries, because in filters they collectively absorb all colors from white light. In color negative reversal film the cyan dyes are used to record red components, magenta dye for green components and the yellow dye for blue components. As a result the processed film displays images which are complementary in color to the recorded object (plot) and negative in tone value. The same dyes are used in positive direct film but in the exposure and developing process the dyes of the subtractive primaries are rejected rather than fixed in the respective emulsion layers.
The consequence in plotting of recording all colors in terms of the three primaries is that any one of the emulsion layers may be called upon to record the colors in excess of its capacity, that is, an emulsion may be over-exposed when the same or similar colors are repeatedly exposed at the same position on the film surface. When the emulsion capacity is exceeded, the colors "bleed" into the surrounding area of the emulsion and produce images which are not clear or accurate representations of the plot pattern. In cases of excessive exposure in one or more of the emulsion layers, the images are not only blurred but also are presented in colors which are not true.
It is, accordingly, a general object of the present invention to provide a method of plotting in color which is accurate and presents clear and true representations of a plot pattern. It is also an object of the invention to provide a method of plotting in color without exceeding the color or exposure capacities of the emulsions recording the colored plot pattern.